Studies are in progress to elucidate several areas related to erythrocyte autoantibodies in man. These include: (a) quantitation of red cell-bound IgG antibody in Coombs-positive and Coombs-negative autoimmune hemolytic anemia, and study of the effect of corticosteroid therapy; and (b) quantitation of autoantigen density of human red cell membranes. These studies utilize quantitative complement fixation (CF) methods or radioimmunoassays. Also under study are applications of these approaches to quantitative measurement of IgG antibody to granulocytes or platelets in clinical disorders.